A major contributor to the inefficiency of an object moving through a fluid is the friction drag or viscous drag that occurs at the boundary layer of the object. Friction drag or viscous drag tends to resist movement of the object through the fluid or movement of fluid over the object. For example, rotating machinery such as a turbine rotating in a fluid is subjected to viscous drag at the boundary layer of the object. In another example, a vehicle such as an aircraft moving through the air is subjected to friction drag or viscous drag at the boundary layer of the vehicle/air interface which tends to impede forward motion of the aircraft.
Included in the prior art are many attempts at reducing viscous drag acting on a surface such as the friction drag acting on an airfoil moving through air or a hydrofoil moving through water. One approach to reducing viscous drag includes forming a plurality of perforations or pores in the surface over which the fluid moves and applying a suction or blowing force to the pores. The application of suction to the pores is based on the principle of removing low energy fluid from the boundary layer of the surface in order to reduce drag. The application of a blowing force to the pores employs the principle of adding higher energy fluid to the boundary layer which delays separation of the boundary layer from the surface. As applied to airfoil or hydrofoil lifting surfaces, delaying separation of the boundary layer increases lift, delays stall at high angles of airfoil attack relative to the moving fluid stream, and thereby improves the efficiency of the lifting surface.
Unfortunately, the application of suction at the pores requires an active vacuum system. Such an active system typically requires the addition of a separate vacuum pump or the addition of a complex series of conduits linking an existing vacuum pump to the pores in the surface. As may be appreciated, the addition of an active vacuum system often results in a heavier system that may result in increased maintenance labor and costs.
Another approach to increasing the efficiency of surfaces moving through a fluid stream includes the use of grooves or riblets on fluid dynamic lifting surfaces. The grooves or riblets may operate to delay flow separation by energizing a boundary layer air stream flowing in proximity to the lifting surface. Grooves or riblets are not intended to reduce drag and may slightly increase viscous drag due to the additional surface area of the lifting surface as a result of the grooves or riblets. Grooves or riblets are provided to energize the boundary layer as a result of boundary layer vortices that form about the trailing edges of the grooves or riblets. The higher energy boundary layer fluid stream separation improves the efficiency of airfoil and hydrofoil lifting surfaces similar to the improved efficiency associated with blowing force technology mentioned above. For smooth surfaces, such vortices move about in the boundary layer air stream and may bounce or deflect off of the lifting surface.
The configuration of the grooves or riblets on the surface may be tailored to the boundary layer air stream and surrounding fluid environment and to the manner in which the lifting surfaces are likely to move relative to the surrounding fluid environment. For example, the grooves or riblets must be oriented along a direction that is generally parallel to the intended direction of movement of the lifting surface relative to the fluid.
As can be seen, there exists a need in the art for a system and method for reducing viscous drag of a surface moving through a fluid and which is preferably a passive system requiring no active components. Furthermore, there exists a need in the art for a system and method for reducing viscous drag of a surface moving through a fluid that performs well when changing directional movement of the lifting surface relative to the fluid. Additionally, there exists a need in the art for a system and method for reducing viscous drag of a surface moving through a fluid that is simple in construction, low in cost, and lightweight.